So my truck just started having a weird idle, it's not rough, it actually runs quite smooth, but it quickly but smoothly ramps the RPM up from 500 to about 1000 RPM, and then quickly but smoothly back down to 500 RPM when in neutral or park. It will continue doing so until put into any gear other than park or neutral.

It has 211,xxx miles on it. About 50,000 miles ago the torque converter was replaced.

Any ideas on what to check, watch, or what might be causing it?

 

time to replace the throttle positioning sensor, with that many miles carbon has built up, simple 2 screws that hold it into throttle body, but oem, these motors don't like aftermarket. after install truck might still do rpm changes until it relearns the correct idle which is 750 rpm